Conversion of heavy stocks into distillates requires substantial boiling range reduction via thermal, catalytic, or hydrocatalytic cracking. One of the major difficulties in such conversion processes is that these heavy petroleum fractions are hydrogen deficient compared to the distillates into which they are to be converted. Their upgrading, particularly catalytic upgrading, is further complicated by substantial levels of heteroatoms (S+N), metals (Ni+V) and asphaltenes. Contaminant levels must be reduced and hydrogen content enriched by processes in which carbon is rejected or hydrogen is added.
Carbon rejection processes necessarily limit liquid yield due to the constraints of stoichiometry. Furthermore, at high temperatures typical of thermal processing, thermodynamic equilibrium favors the formation of coke and high H-content gas. However, thermal carbon rejection processes are well known in the prior art. For example, coking easily converts 100% of a wide range of feedstocks, but is limited to relatively low coker gas oil yield. The products are unstable and require subsequent hydrotreating. Although coking is a flexible process, the quality and marketability of the coke and the degree of treatment required to upgrade the coker gas oil are dependent on feedstock quality. Another thermal process, visbreaking is a low conversion process mostly used to reduce feed viscosity and minimize heavy fuel oil production. Typical conversions are restricted to less than 40% to avoid significant coke formation.
Fluid catalytic cracking is a carbon rejection process which uses a catalyst to maximize gasoline and gas oil quality and yield. However, the most advanced resid crackers are limited to feedstocks with less than about 10% Conradson carbon (CCR) and 60 ppm Ni+V. These specifications exclude almost all heavy stocks, and include only a limited number of higher quality or treated atmospheric resids. Due to stoichiometric limitations, gasoline and distillate yield is less than obtained in catalytic hydrocracking, but is greater than that in coking due to the action of the catalyst.
A number of commercial and exploratory processes for hydrotreating or hydrocracking heavy stocks are known. These are catalytic processes in which hydrogen is added, but feed metals can poison the catalyst and shorten its life. Pressure drop may increase unacceptably in fixed bed processing from accumulation of particulate matter plus coke formed during reaction of the heavy stock, especially at high temperatures. These considerations result in poor cycle length and catalyst life. Moving bed configurations can avoid bed plugging; but fresh catalyst make-up rates, needed to maintain a constant activity level, depend on feedstock quality and are often unacceptably high. Furthermore, these catalytic processes can lead to unselectively high hydrogen consumption, resulting in, for example, hydrogenation of the high-octane aromatic components of gasoline.
Hydrocracking processes such as CANMET and Veba Combicracking can obtain high resid conversions using a relatively wide range of feedstock qualities. Although these processes do not employ a fixed or moving bed of "conventional" catalyst, they do use additives which impart catalytic functionality. Conversion may occur substantially by a thermal mechanism, assisted by the "additive" which may catalytically stabilize the products, preventing their further degradation to coke and gas.
The present invention is a thermal process which uses a solvent to facilitate the addition of hydrogen to the heavy stock while suppressing coke and gas make. The ability of the present invention to add hydrogen in the absence of a catalyst or catalytic "additives" eliminates catalyst fouling and makeup rate problems which are inherent in typical catalytic processes. The current invention permits conversions in excess of 40%, typically 60-80%, with hydrogen addition of up to about 700 SCFB (heavy stock basis), and coke selectivities below about 1 wt %. This allows extended continuous operation with a wide range of feedstocks.